Magnetic resonance imaging (MRI) is an imaging procedure which reconstructs an image from an MR (magnetic resonance) signal generated by magnetically exciting the nuclear spin of an object placed in a static magnetic field with an RF signal of Larmor frequency.
In order to obtain an image at a specific time phase of the cardiac cycle using an MRI apparatus, it is necessary to execute a pulse sequence at a predetermined timing synchronizing synchronized with an ECG (electro cardiogram) waveform obtained from a patient as an object to be imaged, and to perform image reconstruction from the acquired echo signal.
However, in order to obtain the ECG waveform, it is necessary to attach an electrode for detecting the ECG signal to the object in advance of imaging, and therefore, time and labor are required.
In addition, for acquiring suitable ECG signals, it is necessary to adjust the fixing point of the electrode, depending on an individual object. Further, because of a noise that is induced to an ECG waveform under the influence of switching of the gradient magnetic field in imaging, an ECG synchronous imaging sometimes cannot be performed appropriately, and image quality may be deteriorated.
In ECG synchronous imaging, an RF pulse for imaging is applied to the object, while the electrode is attached to the object. Thus, the RF pulse is induced to a loop formed with a signal line for transmitting the ECG waveform, a human body, and an electrode, and there is a danger that the object may be injured with a burn. Such a problem becomes remarkable especially for the high magnetic field MRI apparatus with a large electric power of the RF pulse.
From such a background, instead of detecting an EGG signal from an object, a peripheral pulse gating (PPG) method has been developed. In the PPG method, a sensor is attached to a part of the object, such as a finger, a tiptoe, or an earlobe, and a peripheral pulse signal (a PPG signal) of the object is detected by the sensor to image the object in sync with the peripheral pulse signal.
According to this PPG method, it is not necessary to attach an ECG electrode to an object. Therefore, the problem that a suitable synchronization becomes difficult by disorder of an ECG waveform in a gradient magnetic field is also resolved. In addition, heat generation of an electrode in a MRI apparatus using high magnetic field can also be prevented.
However, it should be noted that, while the imaging by the PPG method is performed in sync with cardiac beats, there exists a certain time delay of the peripheral pulse signal from the actual heart stroke. This time delay is dependent on conditions, such as distance between a detection position of the peripheral signal and the heart, and rigidity of a blood vessel.
Therefore, in a conventional imaging method using simply a peripheral pulse signal, image data corresponding to a specified time phase of the cardiac cycle cannot be obtained, while the artifact resulting from cardiac beat can be reduced.
More specifically, the imaging condition for acquiring data in the specific time phase of the cardiac cycle cannot be determined, before imaging. Further, even after imaging, it cannot be determined which image data corresponds to the specified phase of the cardiac cycle.
Thus, a method has been developed, in which both an ECG signal and a PPG signal are acquired from an object, then, a time delay of the PPG signal to the ECG signal is measured, and then, a cardiac time phase is specified from a PPG signal using the measured time delay.
However, in the method of acquiring both an ECG signal and a PPG signal from an object, while matching of an ECG signal and a PPG signal can be performed, an ECG signal still needs to be acquired. Therefore, the problem that the time and effort are required to attach an electrode for detection of an ECG signal to an object still remains. Further, there still exists the danger that the object may be injured with a burn, resulting from induction of the RF pulse to a loop formed with a signal line for transmitting an ECG waveform, a human body and an electrode.